1. Field of the Invention
The invention concerns an apparatus for measuring the thickness of transparent objects.
2. Description of the Background Art
It is known in the art to use an interferometer for measuring the thickness of transparent objects, particularly a Michelson interferometer. This interferometer has a beam splitter, a so-called coupler that splits a beam generated by a radiation source into two partial beams: a measuring beam and a reference beam. These are guided toward each other after a certain distance, are superposed in the beam splitter, and are then sent back partially into the radiation source or irradiated into an observation detector. If a radiation source that has a radiation with a short coherence length is used, the interference between reference and measuring beam is maintained only when both of their paths are exactly equal in length. This interference can be determined by means of the observation detector.
The beam splitter only initially changes the length of the reference path with a modification speed that is as constant as possible, until the interference of the radiation leaves the reference and measuring paths for determining the distance values of an object with a surface that must reflect merely a reduced fraction of the radiation. Depending upon the modification speed, the radiation frequency f of the radiation source that runs in the reference path undergoes a Doppler displacement according to the relationship ##EQU1##
wherein f.sub.0 represents the original radiation frequency, c represents the light speed, and v represents the speed of modification of the optical path length .tangle-solidup.s--the optical path length modification per time unit.
If an interference occurrence with equal reference and measuring paths can be detected with the observation detector, then the interference pattern is modified according to the above Doppler displacement with the frequency .tangle-solidup.f. .tangle-solidup.f is a time constant if there is a strictly linear modification of the path length difference. If non-linear path length modifications appear, then .tangle-solidup.f varies within the frequency band.
Michelson interferometers wherein the path length modifications are generated in the reference path are known, for example, from EP-A 0,529,603, EP-A 0,443,477, and EP-A 0,449,335. A retroreflector is arranged in each arm of the interferometer in EP-A 0,529,603. Each retroreflector was displaceable by means of a gear that had a toothed belt, so that each front side of the reflector on the surface of a fixed reflector remained parallel even with a distance modification to a fixed reflector. The distance modification took place in such a manner that the optical path length increased in the interference arm, while it decreased in the other. Since both of the drives had to be synchronized, the drive was carried out by means of an electric stepper motor. This arrangement was complicated and also disturbing mechanical oscillations were caused by means of the electric impulse of the stepper motors.
Two retroreflectors per interference arm, which rotated in opposite directions to one another were used in EP-A 0,443,477, plus a further retroreflector that deviated the radiation, as well as a fixed plane level. A synchronization between the rotation of the reflectors was required here also, which was carried out as well by means of the mechanical oscillations of stepper motors.
A thick transparent plane-parallel plate was used in EP-A 0,449,335 for modifying the path length difference in both interference arms. The plate was traveled by the radiation in one direction as well as in the return direction after reflection on the fixed reflector, and the optical path changed according to the inclination of the plate.
The known elements for modifying the path length difference between measuring and reference beam show either a complicated mechanical arrangement that is therefore a vibration-susceptible arrangement and/or a considerable non-linear path difference modification.